FIG. 13 illustrates an example of a conventional liquid treatment apparatus. First electrode 801 and second electrode 802 are disposed in liquid 803 (for example, water), a high voltage pulse is applied between first electrode 801 and second electrode 802 from pulse power supply 804, and liquid 803 is vaporized to generate plasma 805. At this time, pollutants and the like contained in liquid 803 are decomposed by directly coming in contact with plasma 805. At the same time, for example, even in a case where components having oxidizing power such as a hydroxyl radical (OH radical) and hydrogen peroxide are generated and these components react with the pollutants and the like contained in liquid 803, the decomposition treatment progresses. It is known that OH radicals, in particular, have high oxidizing power among the radicals generated by generation of plasma 805 in water, and decomposition treatment of hardly decomposable organic compounds dissolved in liquid 803 can be performed.
However, in a case of the conventional liquid treatment apparatus, since not only a high applied voltage is required for vaporizing liquid 803 but also generation efficiency of plasma 805 is low, there is a problem that it takes a long time to treat liquid 803.
Accordingly, there is known a liquid treatment apparatus in which a gas introduced from the outside is interposed between both electrodes so as to improve the generation efficiency of the plasma while lowering the applied voltage (see PTL 1). In the liquid treatment apparatus (FIG. 14) described in PTL 1, gas 904 (for example, oxygen) is interposed between anode electrode 901 and cathode electrode 902 together with TREATMENT LIQUID 903, and then a pulse voltage is applied to between anode electrode 901 and cathode electrode 902. Plasma is generated in gas 904 by applying the pulse voltage, and decomposition treatment of TREATMENT LIQUID 903 progresses at the contact surface between the plasma and TREATMENT LIQUID 903. According to the liquid treatment described in PTL 1, the applied voltage can be reduced as compared with a case where gas is not interposed, and the plasma can be generated efficiently to treat the liquid.